This invention relates to the treatment of fractured or porous media in well bore-penetrated subterranean formations. In another aspect this invention relates to prolonging mobility control and/or plugging of a well bore-penetrated subterranean formation through applications of thickened aqueous solutions and controlling the gellation of these solutions.
In U.S. Pat. No. 3,762,476 a method is set forth for overcoming the problem of the production of large amounts of water from oil wells and gas wells. Production of large amounts of water constitutes one of the major items of expense in the overall recovery of hydrocarbons from oil and gas wells. It is stated in that patent that since many oil wells produce a gross effluent comprising 80 to 98 percent by volume of water and only 2 to 20 percent by volume of oil that most of the pumping energy in producing a well is expended in lifting water from the well. In such production further expense is incurred by the separation procedures required to recover water-free hydrocarbons and the disposal of the fouled water separated from the hydrocarbons. That patent sets forth the method for injecting into a subterranean formation through a well bore a first aqueous solution thickened with polymer, following this with a crosslinking ionic solution, a brine slug, and then a second thickened aqueous solution, after which the injections are terminated and hydrocarbon fluids are once more recovered from the subterranean formation. By this method the injected polymer solutions comprise polymer that is not previously crosslinked to form a gel until the underground formations have been penetrated by the solution and subsequently by the agents suitable for crosslinking the polymer.
Probably the main reason for using the uncrosslinked polymer in the aqueous solutions is the well-known fact that polymeric solutions that have crosslinked sufficiently to form a gel are notoriously hard to circulate or to inject into an underground formation with sufficient penetration to achieve more than face plugging of permeable formations. I have now discovered that between the extremes of no crosslinking and crosslinking of the polymer solutions sufficient to cause substantial gelling of the solutions there is a definite advantage to injecting partially crosslinked polymer solutions into the underground formation in an operation using consecutive injection of polymer followed by crosslinking and again followed by polymer solution. This process is most effective in highly permeable or fractured reservoirs. The more highly permeable the reservoir the greater the degree of crosslinking of injected polymer that can be effectively used since the pumping power necessary for injecting the polymer into the reservoir depends upon both the permeability of the zones and the degree of crosslinking of the polymer solution.
It is therefore an object of this invention to provide a method for prolonging the mobility control and/or plugging through the application of partially crosslinked polymer solutions into a permeable reservoir with further crosslinking and gelation in situ.
Other objects, aspects and the various advantages of this invention will become apparent upon reading the specification and the appended claims.